For the purposes of the present specification and claims, the term viscous mass is used particularly for a solution containing cellulose and an aqueous tertiary amine-oxide able to be processed to cellulose moulded bodies of any kind, particularly fibers and films.
Tertiary amine-oxides have been known as alternative solvents for cellulose. It is known for instance from U.S. Pat. No. 2,179,181 that tertiary amine-oxides are capable of dissolving cellulose without derivatisation and that from these solutions cellulose moulded bodies, such as fibers, may be produced by precipitation. From EP-A-0 553 070 of the present assignee, further tertiary amine-oxides are known. In the following, all tertiary amine-oxides capable of dissolving cellulose are meant when, for the sake of simplicity, only NMMO (=N-methylmorpholine-N-oxide) is cited.
As alternative solvents, tertiary amine-oxides are advantageous insofar as the cellulose is dissolved by the NMMO without derivatisation, contrary to the viscose process. Thus the cellulose does not have to be chemically regenerated, the NMMO remains chemically unchanged and passes during its precipitation into the precipitation bath and may be recovered from the latter and reused for the preparation of a new solution. Therefore the NMMO process offers the possibility of a closed solvent cycle. Additionally, NMMO has an extremely low toxicity.
However, when cellulose is dissolved in NMMO, the polymerisation degree of the cellulose decreases. Moreover, particularly the presence of metal ions (such as Fe.sup.3+) leads to radically initiated chain cleavages and thus to a significant degradation of the cellulose and the solvent (Buijtenhuijs et al. (The Degradation and Stabilization of Cellulose Dissolved in N-Methylmorpholin-N-Oxide (NMMO), in "Das Papier", Volume 40, number 12, pages 615-619, 1986).
On the other hand, amine-oxides generally have only a limited thermal stability which varies depending on their structure. Under normal conditions, the monohydrate of NMMO is present as a white crystalline solid, which melts at 72.degree. C. Its anhydric compound however melts at no less than 172.degree. C. When heating the monohydrate, intense discoloration will occur from 120.degree./130.degree. C. up. From 175.degree. C. up, an exothermal reaction is initiated, the molten mass being completely dehydrated and great amounts of gas developing which eventually lead to an explosion, the temperatures rising to far over 250.degree. C.
It is known that metallic iron and copper and particularly their salts significantly reduce the decomposition temperature of NMMO, while the decomposition rate is simultaneously increased.
Moreover, additionally to the problems mentioned above, there is another difficulty, i.e. the thermal instability of the NMMO/cellulose solutions themselves. This means that at the elevated processing temperatures (approximately 110.degree.-120.degree. C.), uncontrollable decomposition processes are initiated in the solutions which due to the development of gases may lead to strong deflagrations, fires and even explosions.
To release the excess pressure in pipes produced in the decomposition processes mentioned above, a pipe element having a predetermined breaking point as portion of the pipe wall is known from U.S. Pat. No. 5, 337,776. This predetermined breaking point is provided as a bursting disk. Due to the incorporation of the bursting disk into the pipe wall however, the heating jacket of the pipe is interrupted and therefore the transported mass will cure at the unheated surface of the bursting disk if the transported mass cools down below the solidification point. Another reason for which this solidificated mass will stick to the bursting disk and will not be transported along with the other mass is the reduced rate at the wall. This does not only impair the purpose of the bursting disk, i.e. its timely response, but also a contamination of transported mass will occur if decomposition products are produced in the mass deposited at the bursting disk. This may be the case for instance in solutions of cellulose in tertiary amine-oxides.
Moreover, when incorporating bursting disks into a pipe wall, they have to be dimensioned taking into account the required relief section, which may be calculated by those skilled in the art from the maximum pressure increase rate of the mass system. This means that bursting disk sections which fit to the pipe volume have to be chosen and incorporated. It would be advantageous however to employ bursting disks as small as possible, since the pipe diameter they occupy is reduced and the curvature of the bursting disk diameter adjusts well to the curvature of the pipe wall.
Therefore, an optimum has to be devised whereby the flow rate in the pipe interior is adjusted to the highly viscous flow behaviour by means of the inner diameter of the pipe. Thus, when the mass is highly viscous, reduced shearing rates in the pipe are attained, which is equivalent to a suitably big pipe diameter. When such a pipe is protected by means of a bursting disk arranged level to the pipe interior according to the method known from U.S. Pat. No. 5, 337,776, this will occupy an unnecessarily big circumferencial area of the pipe for the holding device of the bursting disk, leading to the problems described above.
According to U.S. Pat. No. 5,337,776, the bursting disk is adjusted to the pipe. This however is not necessary because of the maximum pressure generating rate of the unstable mass system. This means that a bursting disk having an optimized diameter should be employed.
When due to a higher relief a bigger bursting disk section is required, a poorer adjustment will result, since the pipe circumenference occupied by the bursting disk is big and thus the flow behaviour in the pipe may be deteriorated.